Two enormous, gamma-ray-emitting structures are bubbling out of the center of our galaxy. And astronomers have no idea what caused them.

These bubbles, which stretch an astonishing 25,000 light years above and below the galactic plane, are invisible to the naked eye. But astronomers working with data from the Fermi space telescope, which detects gamma rays, were able to see the structures. Though our galaxy is bathed in a light haze of gamma rays, these structures stood out sharply - gamma rays were zooming out of them at a tremendous rate, and the bubbles also appeared to have sharply defined edges.

Doug Finkbeiner, an astronomer at the Harvard-Smithsonian Center for Astrophysics, was the first researcher to see the bubbles. He told NASA:

What we see are two gamma-ray-emitting bubbles that extend 25,000 light-years north and south of the galactic center. We don't fully understand their nature or origin.

He speculates that the structures could be millions of years old.

So what caused them? According to NASA:

One possibility includes a particle jet from the supermassive black hole at the galactic center. In many other galaxies, astronomers see fast particle jets powered by matter falling toward a central black hole. While there is no evidence the Milky Way's black hole has such a jet today, it may have in the past. The bubbles also may have formed as a result of gas outflows from a burst of star formation, perhaps the one that produced many massive star clusters in the Milky Way's center several million years ago.

"In other galaxies, we see that starbursts can drive enormous gas outflows," said David Spergel, a scientist at Princeton University in New Jersey. "Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics."

Is our galaxy uniquely explosive? Was there a galactic disaster that shaped it?

Maybe our galaxy is where all the advanced civilizations dump their dangerous trash, and what we're seeing is the intergalactic equivalent of a toxic plume.